The digital implementation of a line card described in the above identified co-pending application, includes a transmit path having an impedance selector on the line side, an analog gain/attenuator, e.g. a Micro Linear ML2003, a CODEC for providing .mu.-law compressed data, e.g. a Motorola MC145503 and a digital signal processor (DSP), e.g. a Texas Instruments TMS320E17. The DSP controls the impedance selector and the gain/attenuator in accordance with provisioned settings of impedance and transmit level. The DSP also performs digital equalization. Idle channel noise is of particular concern in the transmit path when digital equalization is used. Idle channel noise is a problem because the .mu.-law codec generates 14-16 dBrncO of idle channel noise due to quantization alone. The digital equalizer of the copending application provides up to 16 dB of slope and the idle channel noise is increased proportionally to the amount of equalizer gain. Thus, the idle channel noise could be as high as 32 dBrncO.
Any solution to the problem of idle channel noise must reduce the noise to an acceptable level, e.g. 20 dBrncO, and maintain amplitude tracking, quantization distortion, and quality of speech and data transmission at specified levels. Noise reduction techniques such as using squelch and quantization expansion were found not to meet these requirements.